[S-CG44] Rheology, fracture and friction in Earth and planetary sciences

[SCG44-02] Deformation structure of the uppermost mantle preserved in the Horoman peridotite complex, Hokkaido

*Kazuki Matsuyama ¹, Katsuyoshi Michibayashi¹ (1.Department of Earth and Planetary Sciences, GSES, Nagoya University)

Keywords:Peridotite, Crystallographic preferred orientation, The Horoman peridotite complex

The Horoman peridotite complex is a large peridotite massif characterized by various microstructures and less serpentinization, and many mantle studies have been conducted (e.g., Takazawa et al. 1999, Ozawa 2004, Morishita and Arai 2003). In this study, peridotite samples were collected from 55 sites in the Horoman olivine body, and grain shape parameters (grain size, area, and long axis) and crystallographic preferred orientation (CPO) of olivine and orthopyroxene were measured by electron backscattered diffraction (EBSD) using a scanning electron microscope (SEM). Frequency distributions of grain shape parameters were generated from the EBSD dataset and compared among samples. Then olivine CPOs were distinguished quantitatively using a Vp-Flinn diagram (Michibayashi et al. 2016). In addition, the shear senses recorded in peridotite were reconstructed using the the obliquity between the lineation and the olivine CPO. The mean grain sizes of the major constituent minerals are 119 µm-330 µm for olivine and 105 µm-378 µm for orthopyroxene. The microstructures were tentatively classified based on the characteristics of the frequency distribution of grain shape. The olivine CPO was classified into three types, A, E, and AG, based on the Vp-Flinn diagram. There was a correlation between olivine CPO and microstructure, with A type characterized by porphyroclastic texture, E type by mylonite to porphyroclastic texture, and AG type by equigranular texture. Besides, the olivine CPO types showed a continuous distribution of E, A, and AG type the southern to northern part of the Horoman peridotite complex (Matsuyama and Michibayashi 2023). The formation of olivine CPO is generally controlled by the physical conditions during plastic deformation (Mainprice 2007). Therefore, the concentration of olivine with E type CPO in the southern part of the complex (corresponding to the lowest part) suggests that a local water infiltration occurred during the movement of the basal fault (Katayama et al. 2004, Matsuyama and Michibayashi 2023). This is consistent with the subsurface structure inferred from seismic velocity/electrical resistivity surveys conducted in the southern end of Hokkaido (Ichihara et al. 2016). It suggests that the E type CPO was formed by dehydration from the subducting Pacific slab. The microstructural and CPO changes from the lower to the upper part of the complex, and the distribution of the reconstructed shear senses suggests that the ascent history of the Horoman peridotite complex could be divided into four stages, integrated with the deformation history of the Hidaka Metamorphic Belt (Toyoshima et al. 1997): (1) upward thrusting of the lower part due to southward movement and water infiltration, (2) upward thrusting of the upper part due to southward movement, (3) westward transpression movement, and (4) local structural modification by duplex development.

References
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